1. Field of the Invention
The present invention relates generally to the measurement of process characteristics of melted materials and pertains, more specifically, to the on-line measurement of such characteristics as the viscosity of polymer melts for purposes of monitoring and controlling of manufacturing processes involving molten plastics.
2. Description of the Prior Art
Rheological testing equipment has been available for a very long time in conducting laboratory measurements of certain important characteristics of polymer melts used in various manufacturing processes. Thus, such properties as viscosity and melt flow index are being measured in the laboratory with increasing accuracy. More recently, efforts have been directed toward the measurement of these characteristics on-line, during the manufacturing process itself, in order to provide constant, closer control over the quality of the melt utilized in the process. On-line measurement requires equipment which not only is relatively easy to use and maintain, but which is rugged enough to withstand the operating conditions to which the equipment will be exposed. In order to be effective, the equipment must be responsive, and must avoid disturbing the manufacturing process being monitored.
Among the more successful on-line rheometers available currently are capillary rheometers which divert a portion of the polymer melt from the main stream of molten plastic, conduct measurements on the diverted melt, and then simply purge the melt out to the atmosphere, hence called A on-line rheometer. Also, rheometers have been developed in which the diverted melt is returned to the main stream, hence called A at(or in)-line) rheometer, thereby eliminating additional steps associated with the purge stream. Both on-line and in-line rheometers usually employ a first metering pump, such as a gear pump, to feed a capillary passage with a controlled flow of the diverted melt, and in case of in-line, a second metering pump to return the diverted melt to the main stream. Pressure drop along the capillary passage is measured and the temperature of the diverted melt is closely controlled with an independent heating or cooling arrangement in order to measure viscosity, as a function of the measure of the pressure drop, to gain the information necessary to control the process.
For a more complete understanding of the state of the technology with respect to rheological testing, consider some instances of known methods and apparatus as reflected by the patent literature. U.S. Pat. No. 4,817,416 to Blanch et al. discloses a system for making in-line rheological measurements utilizing a rheometer of the type in which a first metering pump delivers diverted melt from a process main stream to a capillary passage and a second metering pump returns the diverted melt from the capillary passage to the process main stream and the viscosity of the diverted melt is measured by controlling the rate of flow of the melt to maintain constant the pressure drop between spaced apart locations along the capillary passage and measuring the temperature of the melt in the capillary passage. The measurements may be made while controlling the speed of the second metering pump independent of the speed of the first metering pump to maintain the pressure at the exit of the capillary passage essentially constant. The capillary passage is placed in close proximity to the process main stream for maintaining a relatively short residence time during which the diverted melt resides outside the process main stream so as to attain a relatively quick response to changes in the measured viscosity.
U.S. Pat. No. 4,449,395 to Kurtz et al. discloses a system for testing thermoplastic material according to which a fractional, continuous, molten and flowable sample of the material is passed as a stream to and successively through each of either a controllable pumping zone maintained at constant temperature and pressure or controllable pressure zone maintained at constant flow rate and temperature, and a die zone to form a continuous strand of the material. The viscosity of the material in the pumping and die zones is measured. A measurement of elasticity is provided by obtaining a measurement of percentage of cross-section area swell in the strand in passage from the die zone over a constant length distance downstream of a point of mark sensing. The viscosity measurement is combined with the elasticity measurement to provide fuller rheological characterization data for the material.
U.S. Pat. No. 4,403,502 to Lindt discloses a motionless viscometer and associated method for resinous materials including polymeric foams and non-resinous materials includes a reservoir section, a receiver section and an interposed tube. As the foam is expanded, the axial pressure within the connecting tube is measured as is the rate of rise of the foam within the receiver element. Shear viscosity and density as functions of time may be determined.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.